The need for the effective use of limited energy resources, as well as the need for energy saving to prevent global warming, has hitherto become a matter of public awareness. To date energy needs have been served by means of thermal power generation; that is, by converting heat energy into electric energy. However, considering that coal and oil resources required for thermal power generation are limited in recoverable reserves, the demand for an alternative to such resources has augmented as a natural consequence. As one of new energy resources, attention is being given to a fuel cell for effecting chemical power generation with use of hydrogen for fuel.
A typical fuel cell is composed of two electrodes, namely an anode and a cathode, having sandwiched therebetween an electrolyte. In the cathode, supplied hydrogen is ionized to form hydrogen ion which travels through the electrolyte toward the anode. In the anode, supplied oxygen and the hydrogen ion having passed through the electrolyte react with each other to form water. As the electrons generated in accompaniment with the ionization of hydrogen are moved from the cathode to the anode through wiring, electric current is developed, thus generating electricity.
The fuel cells are classified into four types depending upon the kind of electrolyte: a solid oxide type fuel cell (SOFC) using ion conductive ceramics as an electrolyte; a solid polymer electrolyte type fuel cell (PEFC) using a hydrogen-ion conductive polymeric membrane as an electrolyte; a phosphoric acid type fuel cell (PAFC) using phosphoric acid in highly concentrated form as an electrolyte; and a molten carbonate type fuel cell (MCFC) using alkaline metal carbonate as an electrolyte. Recently, there has been brisk development on the solid polymer electrolyte fuel cell (PEFC) in particular because of its relatively low operating temperature (80° C.).
The solid polymer electrolyte fuel cell is mainly composed of an electrolyte layer, a separator, and a power collector plate. The electrolyte layer has a catalytic electrode formed on its surface. On both sides of the electrolyte layer is disposed the separator so as to sandwich the electrolyte layer. The separator is provided with channels for the supply of hydrogen and oxygen. The power collector plate serves to collect electricity generated in the electrode. Not only the electrolyte layer but also the separator has been improved upon on a repeated basis.
The requirements to be fulfilled by the separator include: high electrical conductivity; high hermeticity against fuel gas and oxidizer gas; and high resistance to corrosion by oxidation-reduction reaction products of hydrogen as well as oxygen.
In order to constitute a separator such as that which satisfies the above stated requirements, the following materials have been used. One of the most frequently used materials is fine-grained carbon which is excellent in electrical conductivity, corrosion resistance, and mechanical strength, and is also higher in workability and lighter in weight. However, the fine-grained carbon is susceptible to oscillation and shock and needs to be subjected to cutting process, which leads to an undesirable increase in the processing cost. It is also necessary to perform additional treatment thereon to attain impermeability to gaseous substances.
As synthetic resin materials, heat-hardening resin such as phenol resin and epoxy resin has been in general use. The synthetic resin, although it is advantageous in terms of cost reduction, offers poor dimensional stability and low electrical conductivity.
Metal materials have also been coming into wider use from the standpoints of electrical conductivity, workability, and hermeticity. In general, titanium and stainless are used. However, the negative side is that metal is susceptible to corrosion, and, in a separator made of metal, metal ion tends to be taken in an electrolyte membrane, which results in deterioration in ion conductivity. In order to avoid this, the separator needs to have its surface plated with gold.
The last example is rubber materials. For example, ethylene-propylene-diene rubber is preferably used. Rubber is low in gas permeability but high in sealability.
Japanese Unexamined Patent Publication JP-A 8-180883 (1996) discloses a solid polymer electrolyte fuel cell. This solid polymer electrolyte fuel cell employs, as a separator, a thin sheet made of such a metal material as lends itself to passivation in an atmospheric environment, for example, stainless steel or titanium alloy. The metal thin sheet is processed into a separator of predetermined configuration by means of press working.
Moreover, Japanese Unexamined Patent Publication JP-A 2002-175818 discloses a separator designed for use in a fuel cell. This fuel cell separator has a rib formed at its outer edge to provide high rigidity, whereby the separator can be prevented from being warped when held by a sealing material.
Further, Japanese Unexamined Patent Publication JP-A 2003-297383 also discloses a separator designed for use in a fuel cell. This fuel cell separator is constituted by a metal base sheet which has, on its one surface at least, a first resin layer and a second resin layer formed of an admixture of resin and an electrically conductive filler. The first resin layer exhibits a volume resistivity of 1.0 Ω·cm or below. The second resin layer is smaller in volume resistivity than the first resin layer. In this way, the separator succeeds in providing enhanced power collecting capability, moldability, strength, and corrosion resistance.
The separator constituted by a metal sheet, although it is excellent in workability, is susceptible to corrosion under the influence of oxygen gas. In such a separator, metal ion is taken in an electrolyte membrane, which results in deterioration in ion conductivity. In order to avoid this, the separator needs to have its surface plated with gold.
Moreover, the separator of conventional design has its outer periphery sealed with a sealing material such as an O-ring to prevent leakage of hydrogen gas, oxygen gas, and coolant.
In the conventional constructions disclosed in JP-A 8-180883 (1996) and JP-A 2002-175818, a gasket is disposed in the vicinity of the separator to prevent leakage of reaction gas and coolant fluid.
As described hereinabove, in the conventional fuel cells, there is a need to interpose a sealing material between the outer periphery of the separator and the cell. Furthermore, in terms of manufacturing process steps, after processing the separator into a desired shape, an additional step is required to fix a sealing material to the outer periphery of the separator or to form a sealing material by means of die molding, with the separator placed as a core.